Electrosurgical generators are employed by surgeons in conjunction with an electrosurgical instrument to cut, coagulate, desiccate and/or seal patient tissue. High frequency electrical energy, e.g., radio frequency (RF) energy, is produced by the electrosurgical generator and applied to the tissue by an electrosurgical tool. Both monopolar and bipolar configurations are commonly used during electrosurgical procedures.
Electrosurgical techniques and instruments can be used to coagulate small diameter blood vessels or to seal large diameter vessels or tissue, e.g., veins and/or soft tissue structures, such as lung, and intestine. A surgeon can cauterize, coagulate/desiccate and/or simply reduce or slow bleeding, by controlling the intensity, frequency and duration of the electrosurgical energy applied between the electrodes and through the tissue. For the purposes herein, the term “cauterization” is defined as the use of heat to destroy tissue (also called “diathermy” or “electro-diathermy”). The term “coagulation” is defined as a process of desiccating tissue wherein the tissue cells are ruptured and dried.
“Vessel sealing” or “tissue fusion” is defined as the process of liquefying the collagen and elastin in the tissue so that it reforms into a fused mass with significantly-reduced demarcation between the opposing tissue structures (opposing walls of the lumen). Coagulation of small vessels is usually sufficient to permanently close them while larger vessels or tissue need to be sealed to assure permanent closure. It has been known that different waveforms of electrosurgical energy are suited for different surgical affects, e.g., cutting, coagulation, sealing, blend, etc. For example, the “cutting” mode typically entails generating a continuous sinusoidal waveform in the frequency range of 250 kHz to 4 MHz with a crest factor in the range of 1.4 to 2.0. The “blend” mode typically entails generating a periodic burst waveform with a duty cycle in the range of 25% to 75% and a crest factor in the range of 2.0 to 5.0. The “coagulate” mode typically entails generating a periodic burst waveform with a duty cycle of approximately 10% or less and a crest factor in the range of 5.0 to 12.0. In order to effectively and consistently seal vessels or tissue, a pulse-like waveform is desired.
In order to optimize sealing or tissue fusion without causing unwanted charring of tissue at the surgical site or possibly causing collateral damage to adjacent tissue, e.g., thermal spread, it is necessary to accurately control the output from the electrosurgical generator, e.g., power, waveform, voltage, current, pulse rate, etc. It follows that accurate measurement of the output power of an electrosurgical generator greatly benefits the design, manufacture, and use thereof.
The task of acquiring power data from an electrosurgical generator unit typically involves coupling the RF output of the generator to a dummy load, and manually activating an output power mode and/or level via front panel controls or other actuator. The current value through the load is measured with an RMS voltmeter and recorded manually for each data point along a test sequence. Every data point must then be transferred into a form suitable for design analysis or individual product calibration by a design engineer or line technician. The entire series of measurements may be repeated for different power levels and with different dummy loads. For example, test data may be manually input into a spreadsheet or bench test equipment to calculate load power for each data point. Each power level and mode setting requires at least 20 data points to define a curve with a meaningful level of detail. Typically, at least three power levels are used to define a particular mode. Thus, for each electrosurgical mode, at least 60 data points need to be collected. This means that for an electrosurgical generator that can operate in a cut mode, a blend mode, a coagulation mode, and a sealing mode, 240 data points are required to meet the minimum level of precision required. The result is a time-consuming and labor-intensive product development cycle or manufacturing process which adds considerable cost to the product and negatively impacts time-to-market and margins.
Additionally, most active loads are designed for use with DC outputs. Typical DC active loads use a MOSFET. However, an AC waveform can damage the MOSFET because of the parasitic of the device and the intrinsic body diode. To accommodate the AC waveform, different configurations have been used such as an N-channel and a P-channel device (See FIG. 2B), or a common source configuration. However, in these configurations the device does not behave correctly over the waveform cycle. Further, the drain to source capacitance changes as the drain voltage varies, which induces load variations dependent on the output voltage level. Another approach for an active load to handle the AC waveform was to rectify the voltage into a DC voltage; however this results in unwanted spikes in the waveform as the diodes go through reverse recovery.